A. Field of the Invention
The invention generally concerns carbon nanotubes (CNTs). In particular, a method of making a structured carbon nanotube (CNT) shell or cage material is disclosed. The CNT shell can further include a core or yolk encapsulated within the CNT shell.
B. Description of Related Art
Carbon nanotubes (CNTs) are nanometer-scale tubular-shaped graphene structures that have extraordinary mechanical, chemical, optical and electrical properties (See, Iijima, “Helical microtubules of graphitic carbon”, Nature, 1991, 354:56-58, “Iijima”). By way of example, CNTs have been shown to exhibit good electrical conductivity and tensile strength, including high strain to failure and relatively high tensile modulus. CNTs have also been shown to be highly resistant to fatigue, radiation damage, and heat. These properties make CNTs a material that can be used in a variety of applications (e.g., conductive, electromagnetic, microwave, absorbing, high-strength composites, super capacitor, battery electrodes, catalyst and catalyst supports, field emission displays, transparent conducting films, drug delivery systems, electronic devices, sensors and actuators).
Several different processes for making CNTs have been developed over the years. Generally, the three main methods are: (1) arc discharge method (See, Iijima), (2) laser ablation method (See, Ebbesen et al., “Large-scale Synthesis of Carbon Nanotubes”, Nature, 1992, 358:220), and (3) chemical vapor deposition (CVD) method (See, Li, “Large-scale Synthesis of Aligned Carbon Nanotubes”, Science, 1996, 274:1701). Other CNT production methods have also been developed. For instance, Zhang et al., “Spherical Structures Composed of Multiwalled Carbon Nanotubes: Formation Mechanism and Catalytic Performance” Angew. Chem. Int. ed., 2012, 51:7581-7585, discloses a process to produce a solid CNT monolith as an alternative to the more typical chemical-vapor-deposition (CVD) process and indicates that its process would allow for large scale production of CNTs.
Despite all of the currently available research on CNTs, utilization of their unique properties has yet to be fully realized. This is due, in part, to the structural limitations currently seen with CNT-based materials. In particular, while the above CNT production processes can be used to produce CNTs, these processes are limited and typically do not allow for the preparation of CNTs having desired structural properties. This has led to research in producing structured CNT-based materials such as CNT shells or cages, which can find use in a wide array of applications (e.g., energy storage applications (e.g., secondary batteries) or catalytic chemistry reactions). The currently available processes for producing such structured CNT-based materials, however, can be complicated, time consuming, and expensive. By way of example, Sano et al., “Noncovalent Self-Assembly of Carbon Nanotubes for Construction of ‘Cages’”, Nano Letters, 2002, 2:531-533, discloses a process for making a hollow spherical cage of nested single-walled carbon nanotubes by adsorbing nanotubes onto amine-terminated silica gels in solution, drying the solution and then performing a re-adsorption cycle to grow a nanotube nest layer-by-layer. The silica is removed by etching to give way to produce hollow cages. In another example, Nguyen et al., “Synthesis of sea urchin-like particles of carbon nanotubes directly grown on stainless steel cores and their effect on the mechanical properties of polymer composites”, Carbon, 2010, 48, 2910-2916, describes growing multi-walled carbon nanotubes on spherical stainless steel particles to produce a structure that resembled sea urchins. In still another example, Tang et al., “Hollow Carbon Nanotube Microspheres and Hemimicrospheres”, The Journal of Physical Chemistry C, 2009, 113:1666-1671 describes a layer-by layer (LBL) assembly of multiwalled carbon nanotubes and diazoresin using electrostatic interactions on planar substrates and polystyrene cores. The polystyrene cores were removed by calcination of the structure to produce a hollow multiwalled microsphere.